Cold Rolled Steel Sheet Having High Yield Ratio And Less Anisotropy, Process For Producing The Same

ABSTRACT

Disclosed herein is a Nb—Ti composite IF steel in which fine precipitates, such as CuS precipitates, having a size of 0.2 μm or less are distributed. The distribution of fine precipitates in the Nb—Ti composite IF steel enhances the yield strength and lowers the in-plane anisotropy index. The nanometer-sized precipitates allow the formation of minute crystal grains. As a result, dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, which is advantageous in terms of room-temperature non-aging properties and bake hardenability.

TECHNICAL FIELD

The present invention relates to niobium (Nb) based interstitial free (IF) cold rolled steel sheets that are used as materials for automobiles, household electronic appliances, etc. More particularly, the present invention relates to IF cold rolled steel sheets with high yield ratio whose in-plane anisotropy is lowered due to the distribution of fine precipitates, and a method for producing such steel sheets.

BACKGROUND ART

In general, cold rolled steel sheets for use in automobiles and household electronic appliances are required to have excellent room-temperature aging resistance and bake hardenability, together with high strength and superior formability.

Aging is a strain aging phenomenon that arises from hardening caused by dissolved elements, such as C and N, fixed to dislocations. Since aging causes defect, called “stretcher strain”, it is important to secure excellent room-temperature aging resistance.

Bake hardenability means increase in strength due to the presence of dissolved carbon after press formation, followed by painting and drying, by leaving a slight small amount of carbon in a solid solution state. Steel sheets with excellent bake hardenability can overcome the difficulties of press formability resulting from high strength.

Room-temperature aging resistance and bake hardenability can be imparted to aluminum (Al)-killed steels by batch annealing of the Al-killed steels. However, extended time of the batch annealing causes low productivity of the Al-killed steels and severe variation in steel materials at different sites. In addition, Al-killed steels have a bake hardening (BH) value (a difference in yield strength before and after painting) of 10-20 MPa, which demonstrates that an increase in yield strength is low.

Under such circumstances, interstitial free (IF) steels with excellent room-temperature aging resistance and bake hardenability have been developed by adding carbide and nitride-forming elements, such as Ti and Nb, followed by continuous annealing.

For example, Japanese Patent Application Publication No. Sho 57-041349 describes an enhancement in the strength of a Ti-based IF steel by adding 0.4-0.8% of manganese (Mn) and 0.04-0.12% of phosphorus (P). In very low carbon IF steels, however, P causes the problem of secondary working embrittlement due to segregation in grain boundaries.

Japanese Patent Application Publication No. Hei 5-078784 describes an enhancement in strength by the addition of Mn as a solid solution strengthening element in an amount exceeding 0.9% and not exceeding 3.0%.

Korean Patent Application Publication No. 2003-0052248 describes an improvement in secondary working embrittlement resistance as well as strength and workability by the addition of 0.5-2.0% of Mn instead of P, together with aluminum (Al) and boron (B).

Japanese Patent Application Publication No. Hei 10-158783 describes an enhancement in strength by reducing the content of P and using Mn and Si as solid solution strengthening elements. According to this publication, Mn is used in an amount of up to 0.5%, Al as a deoxidizing agent is used in an amount of 0.1%, and nitrogen (N) as an impurity is limited to 0.01% or less. If the Mn content is increased, the plating characteristics are worsened.

Japanese Patent Application Publication No. Hei 6-057336 discloses an enhancement in the strength of an IF steel by adding 0.5-2.5% of copper (Cu) to form ε-Cu precipitates. High strength of the IF steel is achieved due to the presence of the ε-Cu precipitates, but the workability of the IF steel is worsened.

Japanese Patent Application Publication Nos. Hei 9-227951 and Hei 10-265900 suggest technologies associated with improvement in workability or surface defects due to carbides by the use of Cu as a nucleus for precipitation of the carbides. According to the former publication, 0.005-0.1% of Cu is added to precipitate CuS during temper rolling of an IF steel, and the CuS precipitates are used as nuclei to form Cu—Ti—C—S precipitates during hot rolling. In addition, the former publication states that the number of nuclei forming a {111} plane parallel to the surface of a plate increases in the vicinity of the Cu—Ti—C—S precipitates during recrystallization, which contributes to an improvement in workability. According to the latter publication, 0.01-0.05% of Cu is added to an IF steel to obtain CuS precipitates and then the CuS precipitates are used as nuclei for precipitation of carbides to reduce the amount of dissolved carbon (C), leading to an improvement in surface defects. According to the prior art, since coarse CuS precipitates are used during production of cold rolled steel sheets, carbides remain in the final products. Further, since emulsion-forming elements, such as Ti and Zr, are added in an amount greater than the amount of sulfur (S) in an atomic weight ratio, a main portion of the sulfur (S) reacts with Ti or Zr rather than Cu.

On the other hand, Japanese Patent Application Publication Nos. Hei 6-240365 and Hei 7-216340 describe the addition of a combination of Cu and P to improve the corrosion resistance of baking hardening type IF steels. According to these publications, Cu is added in an amount of 0.05-1.0% to ensure improved corrosion resistance. However, in actuality, Cu is added in an excessively large amount of 0.2% or more.

Japanese Patent Application Publication Nos. Hei 10-280048 and Hei 10-287954 suggest the dissolution of carbosulfide (Ti—C—S based) in a carbide at the time of reheating and annealing to obtain a solid solution in crystal grain boundaries, thereby achieving a bake hardening (BH) value (a difference in yield strength before and after baking) of 30 MPa or more.

According to the aforementioned publications, strength is enhanced by strengthening solid solution or using ε-Cu precipitates. Cu is used to form ε-Cu precipitates and improve corrosion resistance. In addition, Cu is used as a nucleus for precipitation of carbides. No mention is made in these publications about an increase in high yield ratio (i.e. yield strength/tensile strength) and a reduction in in-plane anisotropy index. If the tensile strength-to-yield strength ratio (i.e. yield ratio) of an IF steel sheet is high, the thickness of the IF steel sheet can be reduced, which is effective in weight reduction. In addition, if the in-plane anisotropy index of an IF steel sheet is low, fewer wrinkles and ears occur during processing and after processing, respectively.

[Disclosure] [Technical Problem]

An object of certain embodiments of the invention is to provide Nb based IF cold rolled steel sheets and a method for producing such steel sheets that are capable of achieving a high yield ratio and a low in-plane anisotropy index.

Another object of certain embodiments of the invention is to provide a method for producing such steel sheets.

[Technical Solution]

According to an aspect of the present invention, there is provided a cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled steel sheet having a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies a relationship: 1≦(Cu/63.5)/(S/32)≦30, and the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.

In an embodiment of the present invention, the cold rolled steel sheet has a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies a relationship: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.

In another embodiment of the present invention, the cold rolled steel sheet has a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies a relationship: 1≦(Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.

In further another embodiment of the present invention, the cold rolled steel sheet has a composition comprising: 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.

According to another aspect of the present invention, there is provided a cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled sheet having a composition comprising: 0.01% or less C, 0.08% or less S, 0.1% or less Al, 0.004% or less N, 0.2% P, 0.0001 to 0.002% of B, 0.002 to 0.04% of Nb, at least one kind selected from 0.01 to 0.2% of Cu, 0.01 to 0.3% of Mn and 0.004 to 0.2% of N, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, where the N content is 0.004% or more, and the steel sheet comprises at least one kind selected from (Nn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.

For room-temperature non-aging properties, the C and Nb contents satisfies a relationship, by weight: 0.8≦(Nb/93)/(C/12)≦5.0. In addition, for bake hardenability, solute carbon (Cs) is from 5 to 30, where Cs=(C−Nb×12/93)×10,000.

Depending on the design of the compositions, the cold rolled steel sheets of the present invention have characteristics of soft cold rolled steel sheets of the order of 280 MPa and high-strength cold rolled steel sheets of the order of 340 MPa or more.

When the content of P in the compositions of the present invention is 0.015% or less, soft cold rolled steel sheets of the order of 280 MPa are produced. When the soft cold rolled steel sheets further contain at least one solid solution strengthening element selected from Si and Cr, or the P content is in the range of 0.015-0.2%, a high strength of 340 MPa or more is attained. The P content in the high-strength steels containing P alone is preferably in the range of 0.03% to 0.2%. The Si content in the high-strength steels is preferably in the range of 0.1 to 0.8%. The Cr content in the high-strength steels is preferably in the range of 0.2 to 1.2. In the case where the cold rolled steel sheets of the present invention contain at least one element selected from Si and Cr, the P content may be freely designed in an amount of 0.2% or less.

For better workability, the cold rolled steel sheets of the present invention may further contain 0.01-0.2 wt % of Mo.

According to further another aspect of the present invention, there is provided a method for producing the cold rolled steel sheets, the method comprising steps of reheating a slab satisfying one of the compositions to a temperature of 1,100° C. or higher; hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet; cooling the hot rolled steel sheet at a rate of 300° C./min.; winding the cooled steel sheet at 700° C. or lower; cold rolling the wound steel sheet; and continuously annealing the cold rolled steel sheet

BEST MODE

The present invention will be described in detail below.

Fine precipitates having a size of 0.2 μm or less are distributed in the cold rolled steel sheets of the present invention. Examples of such precipitates include MnS precipitates, CuS precipitates, and composite precipitates of MnS and CuS. These precipitates are referred to simply as “(Mn,Cu)S”.

The present inventors have found that when fine precipitates are distributed in Nb based IF steels, the yield strength of the IF steels is enhanced and the in-plane anisotropy index of the IF steels is lowered, thus leading to an improvement in workability. The present invention has been achieved based on this finding. The precipitates used in the present invention have drawn little attention in conventional IF steels. Particularly, the precipitates have not been actively used from the viewpoint of yield strength and in-plane anisotropy index.

Regulation of the components in the Nb based IF steels is required to obtain (Mn,Cu)S precipitates and/or AlN precipitates. If the IF steels contain Ti, Nb, Zr and other elements, S preferentially reacts with Ti and Zr. Since the cold rolled steel sheets of the present invention are Nb added IF steels, S for (Mn,Cu)S precipitates through content regulation of Cu an Mn. N is precipitated into AlN through content regulation of Al and N.

The fine precipitates thus obtained allow the formation of minute crystal grains. Minuteness in the size of crystal grains relatively increases the proportion of crystal grain boundaries. Accordingly, the dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, thus achieving excellent room-temperature non-aging properties. Since the dissolved carbon present within the crystal grains can more freely migrate, it binds to movable dislocations, thus affecting the room-temperature aging properties. In contrast, the dissolved carbon segregated in stable positions, such as in the crystal grain boundaries and in the vicinity of the precipitates, is activated at a high temperature, for example, a temperature for painting/baking treatment, thus affecting the bake hardenability.

The fine precipitates distributed in the steel sheets of the present invention have a positive influence on the increase of yield strength arising from precipitation enhancement, improvement in strength-ductility balance, in-plane anisotropy index, and plasticity anisotropy. To this end, the fine (Mn,Cu)S precipitates and AlN precipitates must be uniformly distributed. According to the cold rolled steel sheets of the present invention, contents of components affecting the precipitation, composition between the components, production conditions, and particularly cooling rate after hot rolling, have a great influence on the distribution of the fine precipitates.

The constituent components of the cold rolled steel sheets according to the present invention will be explained.

The content of carbon (C) is preferably limited to 0.01% or less.

Carbon (C) affects the room-temperature aging resistance and bake hardenability of the cold rolled steel sheets. When the carbon content exceeds 0.01%, the addition of the expensive agents Nb and Ti is required to remove the remaining carbon, which is economically disadvantageous and is undesirable in terms of formability. When it is intended to achieve room-temperature aging resistance only, it is preferred to maintain the carbon content at a low level, which enables the reduction of the amount of the expensive agents Nb and Ti added. When it is intended to ensure desired bake hardenability, the carbon is preferably added in an amount of 0.001% or more, and more preferably 0.005% to 0.01%. When the carbon content is less than 0.005%, room-temperature aging resistance can be ensured without increasing the amounts of Nb and Ti.

The content of copper (Cu) is preferably in the range of 0.01-0.2%.

Copper serves to form fine CuS precipitates, which make the crystal grains fine. Copper lowers the in-plane anisotropy index of the cold rolled steel sheets and enhances the yield strength of the cold rolled steel sheets by precipitation promotion. In order to form fine precipitates, the Cu content must be 0.01% or more. When the Cu content is more than 0.2%, coarse precipitates are obtained. The Cu content is more preferably in the range of 0.03 to 0.2%.

The content of manganese (Mn) is preferably in the range of 0.01-0.3%.

Manganese serves to precipitate sulfur in a solid solution state in the steels as MnS precipitates, thereby preventing occurrence of hot shortness caused by the dissolved sulfur, or is known as a solid solution strengthening element. From such a technical standpoint, manganese is generally added in a large amount. The present inventors have found that when the manganese content is reduced and the sulfur content is optimized, very fine MnS precipitates are obtained. Based on this finding, the manganese content is limited to 0.3% or less. In order to ensure this characteristic, the manganese content must be 0.01% or more. When the manganese content is less than 0.01%, i.e. the sulfur content remaining in a solid solution state is high, hot shortness may occur. When the manganese content is greater than 0.3%, coarse MnS precipitates are formed, thus making it difficult to achieve desired strength. A more preferable Mn content is within the range of 0.01 to 0.12%.

The content of sulfur (S) is preferably limited to 0.08% or less.

Sulfur (S) reacts with Cu and/or Mn to form CuS and MnS precipitates, respectively. When the sulfur content is greater than 0.08%, the proportion of dissolved sulfur is increased. This increase of dissolved sulfur greatly deteriorates the ductility and formability of the steel sheets and increases the risk of hot shortness. In order to obtain as many CuS and/or MnS precipitates as possible, a sulfur content of 0.005% or more is preferred.

The content of aluminum (Al) is preferably limited to 0.1% or less.

Aluminum reacts with nitrogen (N) to form fine AlN precipitates, thereby completely preventing aging by dissolved nitrogen. When the nitrogen content is 0.004% or more, AlN precipitates are sufficiently formed. The distribution of the fine AlN precipitates in the steel sheets allows the formation of minute crystal grains and enhances the yield strength of the steel sheets by precipitation enhancement. A more preferable Al content is in the range of 0.01 to 0.1%.

The content of nitrogen (N) is preferably limited to 0.02% or less.

When it is intended to use AlN precipitates, nitrogen is added in an amount of up to 0.02%. Otherwise, the nitrogen content is controlled to 0.004% or less. When the nitrogen content is less than 0.004%, the number of the AlN precipitates is small, and therefore, the minuteness effects of crystal grains and the precipitation enhancement effects are negligible. In contrast, when the nitrogen content is greater than 0.02%, it is difficult to guarantee aging properties by use of dissolved nitrogen.

The content of phosphorus (P) is preferably limited to 0.2% or less.

Phosphorus is an element that has excellent solid solution strengthening effects while allowing a slight reduction in r-value. Phosphorus guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled. It is desirable that the phosphorus content in steels requiring a strength of the order of 280 MPa be defined to 0.015% or less. It is desirable that the phosphorus content in high-strength steels of the order of 340 MPa be limited to a range exceeding 0.015% and not exceeding 0.2%. A phosphorus content exceeding 0.2% can lead to a reduction in ductility of the steel sheets. Accordingly, the phosphorus content is preferably limited to a maximum of 0.2%. When Si and Cr are added in the present invention, the phosphorus content can be appropriately controlled to be 0.2% or less to achieve the desired strength.

The content of boron (B) is preferably in the range of 0.0001 to 0.002%.

Boron is added to prevent occurrence of secondary working embrittlement. To this end, a preferable boron content is 0.0001% or more. When the boron content exceeds 0.002%, the deep drawability of the steel sheets may be markedly deteriorated.

The content of niobium (Nb) is preferably in the range of 0.002 to 0.04%.

Nb is added for the purpose of ensuring the non-aging properties and improving the formability of the steel sheets. Nb, which is a potent carbide-forming element, is added to steels to form NbC precipitates in the steels. In addition, the NbC precipitates permit the steel sheets to be well textured during annealing, thus greatly improving the deep drawability of the steel sheets. When the content of Nb added is not greater than 0.002%, the NbC precipitates are obtained in very small amounts. Accordingly, the steel sheets are not well textured and thus there is little improvement in the deep drawability of the steel sheets. In contrast, when the Nb content exceeds 0.04%, the NbC precipitates are obtained in very large amounts. Accordingly, the deep drawability and elongation of the steel sheets are lowered, and thus the formability of the steel sheets may be markedly deteriorated.

To obtain (Mn,Cu)S and AlN precipitates, the Mn, Cu, S, Al and N contents are adjusted within the ranges defined by the following relationships. The respective components indicated in the following relationships are expressed as percentages by weight.

1≦(Cu/63.5)/(S/32)≦30   (1)

Relationship 1 is associated with the formation of (Mn,Cu)S precipitates. To obtain fine CuS precipitates, it is preferred that the value of relationship 1 be equal to or greater than 1. If the value of relationship 1 is greater than 30, coarse CuS precipitates are distributed, which is undesirable. To stably obtain CuS precipitates having a size of 0.2 μm or less, the value of relationship 1 is preferably in the range of 1 to 9, and most preferably 1 to 6. The reason for this limitation is to obtain fine (Mn,Cu)S precipitates.

1≦(Mn/55+Cu/63.5)/(S/32)≦30   (2)

Relationship 2 is associated with the formation of (Mn,Cu)S precipitates, and is obtained by adding a Mn content to Relationship 1. To obtain effective (Mn,Cu)S precipitates, the value of relationship 2 must be 1 or greater. When the value of Relationship 2 is greater than 30, coarse (Mn,Cu)S precipitates are obtained. To stably obtain CuS precipitates having a size of 0.2 μm or less, the value of relationship 2 is preferably in the range of 1 to 9, and most preferably 1 to 6.

1≦(Al/27)/(N/14)≦10   (3)

Relationship 3 is associated with the formation of AlN precipitates. When the value of Relationship 3 is less than 1, aging may take place due to dissolved N. When the value of Relationship 3 is greater than 10, coarse AlN precipitates are obtained, and thus sufficient strength is not obtained. Preferably, the value of relationship 3 is in the range of 1 to 5.

The components of the cold rolled steel sheets according to the present invention may be combined in various ways according to the kind of precipitates to be obtained. For example, the present invention provides a cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled sheet having a composition comprising: 0.01% or less C, 0.08% or less S, 0.1% or less Al, 0.004% or less N, 0.2% P, 0.0001 to 0.002% of B, 0.002 to 0.04% of Nb, at least one kind selected from 0.01 to 0.2% of Cu, 0.01 to 0.3% of Mn and 0.004 to 0.2% of N, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, where the N content is 0.004% or more. Then, the steel sheet comprises at least one kind selected from NnS precipitates, CuS precipitates, composite precipitates of MnS and CuS, and AlN precipitates having an average size of 0.2 μm or less. That is, one or more kinds selected from the group consisting of 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N lead to various combinations of (Mn,Cu)S and AlN precipitates having a size not greater than 0.2 μm.

In the steel sheets of the present invention, carbon is precipitated into NbC and TiC forms. Accordingly, the room-temperature aging resistance and bake hardenability of the steel sheets are affected depending on the conditions of dissolved carbon under which NbC and TiC precipitates are not obtained. Taking into account these requirements, it is most preferred that the Nb, Ti and C contents satisfy the following relationships.

0.8≦(Nb/93)/(C/12)≦5.0   (4)

Relationship 4 is associated with the formation of NbC precipitates to remove the carbon in a solid solution state, thereby achieving room-temperature non-aging properties. When the value of relationship 4 is less than 0.8, it is difficult to ensure room-temperature non-aging properties. In contrast, when the value of relationship 4 is greater than 5, the amounts of Nb and Ti remaining in a solid solution state in the steels are large, which deteriorates the ductility of the steels. When it is intended to achieve room-temperature non-aging properties without securing bake hardenability, it is preferred to limit the carbon content to 0.005% or less. Although the carbon content is more than 0.005%, room-temperature non-aging properties can be achieved when Relationship 4 is satisfied but the amounts of NbC precipitates are increased, thus deteriorating the workability of the steel sheets.

Cs(solute carbon):5-30, where Cs=(C−Nb×12/93)×10,000   (5)

Relationship 5 is associated with the achievement of bake hardenability. Cs, which represents the content of dissolved carbon, and is expressed in ppm. In order to achieve a high bake hardening value, the Cs value must be 5 ppm or more. If the Cs value exceeds 30 ppm, the content of dissolved carbon is increased, making it difficult to attain room-temperature non-aging properties.

It is advantageous that the fine precipitates are uniformly distributed in the compositions of the present invention. It is preferable that the precipitates have an average size of 0.2 μm or less. According to a study conducted by the present inventors, when the precipitates have an average size greater than 0.2 μm, the steel sheets have poor strength and low in-plane anisotropy index. Further, large amounts of precipitates having a size of 0.2 μm or less are distributed in the compositions of the present invention. While the number of the distributed precipitates is not particularly limited, it is more advantageous with higher number of the precipitates. The number of the distributed precipitates is preferably 1×10⁵/mm² or more, more preferably 1×10⁶/mm² or more, and most preferably 1×10⁷/mm² or more. The plasticity-anisotropy index is increased and the in-plane anisotropy index is lowered with increasing number of the precipitates, and as a result, the workability is greatly improved. It is commonly known that there is a limitation in increasing the workability because the in-plane anisotropy index is increased with increasing plasticity-anisotropy index. It is worth noting that as the number of the precipitates distributed in the steel sheets of the present invention increases, the plasticity-anisotropy index of the steel sheets is increased and the in-plane anisotropy index of the steel sheets is lowered. The steel sheets of the present invention in which the fine precipitates are formed satisfy a yield ratio (yield strength/tensile strength) of 0.58 or higher.

When the steel sheets of the present invention are applied to high-strength steel sheets of the order of 340 MPa, they may further contain at least one solid solution strengthening element selected from P, Si and Cr.

The addition effects of P have been previously described, and thus their explanation is omitted.

The content of silicon (Si) is preferably in the range of 0.1 to 0.8%.

Si is an element that has solid solution strengthening effects and shows a slight reduction in elongation. Si guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled. Only when the Si content is 0.1% or more, high strength can be ensured. However, when the Si content is more than 0.8%, the ductility of the steel sheets is deteriorated.

The content of chromium (Cr) is preferably in the range of 0.2 to 1.2%.

Cr is an element that has solid solution strengthening effects, lowers the secondary working embrittlement temperature, and lowers the aging index due to the formation of Cr carbides. Cr guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled and serves to lower the in-plane anisotropy index of the steel sheets. Only when the Cr content is 0.2% or more, high strength can be ensured. However, when the Cr content exceeds 1.2%, the ductility of the steel sheets is deteriorated.

The cold rolled steel sheets of the present invention may further contain molybdenum (Mo).

The content of molybdenum (Mo) in the cold rolled steel sheets of the present invention is preferably in the range of 0.01 to 0.2%.

Mo is added as an element that increases the plasticity-anisotropy index of the steel sheets. Only when the molybdenum content is not lower than 0.01%, the plasticity-anisotropy index of the steel sheets is increased. However, when the molybdenum content exceeds 0.2%, the plasticity-anisotropy index is not further increased and there is a danger of hot shortness.

Production of Cold Rolled Steel Sheets Hereinafter, a process for producing the cold rolled steel sheets of the present invention will be explained with reference to the preferred embodiments that follow. Various modifications of the embodiments of the present invention can be made, and such modifications are within the scope of the present invention.

The process of the present invention is characterized in that a steel satisfying one of the steel compositions defined above is processed through hot rolling and cold rolling to form precipitates having an average size of 0.2 μm or less in a cold rolled sheet. The average size of the precipitates in the cold rolled plate is affected by the design of the steel composition and the processing conditions, such as reheating temperature and winding temperature. Particularly, cooling rate after hot rolling has a direct influence on the average size of the precipitates.

Hot Rolling Conditions

In the present invention, a steel satisfying one of the compositions defined above is reheated, and is then subjected to hot rolling. The reheating temperature is preferably 1,100° C. or higher. When the steel is reheated to a temperature lower than 1,100° C., coarse precipitates formed during continuous casting are not completely dissolved and remain. The coarse precipitates still remain even after hot rolling.

It is preferred that the hot rolling is performed at a finish rolling temperature not lower than the Ar₃ transformation point. When the finish rolling temperature is lower than the Ar₃ transformation point, rolled grains are created, which deteriorates the workability and causes poor strength.

The cooling is preferably performed at a rate of 300° C./min or higher before winding and after hot rolling. Although the composition of the components is controlled to obtain fine precipitates, the precipitates may have an average size greater than 0.2 μm at a cooling rate of less than 300° C./min. That is, as the cooling rate is increased, many nuclei are created and thus the size of the precipitates becomes finer and finer. Since the size of the precipitates is decreased with increasing cooling rate, it is not necessary to define the upper limit of the cooling rate. When the cooling rate is higher than 1,000° C./min., however, a significant improvement in the size reduction effects of the precipitates is not further shown. Therefore, the cooling rate is preferably in the range of 300-1000° C./min.

Winding Conditions

After the hot rolling, winding is performed at a temperature not higher than 700° C. When the winding temperature is higher than 700° C., the precipitates are grown too coarsely, thus making it difficult to ensure high strength.

Cold Rolling Conditions

The steel is cold rolled at a reduction rate of 50-90%. Since a cold reduction rate lower than 50% leads to creation of a small amount of nuclei upon annealing recrystallization, the crystal grains are grown excessively upon annealing, thereby coarsening of the crystal grains recrystallized through annealing, which results in reduction of the strength and formability. A cold reduction rate higher than 90% leads to enhanced formability, while creating an excessively large amount of nuclei, so that the crystal grains recrystallized through annealing become too fine, thus deteriorating the ductility of the steel.

Continuous Annealing

Continuous annealing temperature plays an important role in determining the mechanical properties of the final product. According to the present invention, the continuous annealing is preferably performed at a temperature of 700 to 900° C. When the continuous annealing is performed at a temperature lower than 700° C., the recrystallization is not completed and thus a desired ductility cannot be ensured. In contrast, when the continuous annealing is performed at a temperature higher than 900° C., the recrystallized grains become coarse and thus the strength of the steel is deteriorated. The continuous annealing is maintained until the steel is completely recrystallized. The recrystallization of the steel can be completed for about 10 seconds or more. The continuous annealing is preferably performed for 10 seconds to 30 minutes.

[Mode for Invention]

The present invention will now be described in more detail with reference to the following examples.

The mechanical properties of steel sheets produced in the following examples were evaluated according to the ASTM E-8 standard test methods. Specifically, each of the steel sheets was machined to obtain standard samples. The yield strength, tensile strength, elongation, plasticity-anisotropy index (r_(m) value) and in-plane anisotropy index (Δr value), and the aging index were measured using a tensile strength tester (available from INSTRON Company, Model 6025). The plasticity-anisotropy index r_(m) and in-plane anisotropy index (Δr value) were calculated by the following equations: r_(m)=(r₀+2r₄₅+r₉₀)/4 and Δr=(r₀−2r₄₅+r₉₀)/2, respectively.

The aging index of the steel sheets is defined as a yield point elongation measured by annealing each of the samples, followed by 1.0% skin pass rolling and thermally processing at 100° C. for 2 hours. The bake hardening (BH) value of the standard samples was measured by the following procedure. After a 2% strain was applied to each of the samples, the strained sample was annealed at 170° C. for 20 minutes. The yield strength of the annealed sample was measured. The BH value was calculated by subtracting the yield strength measured before annealing from the yield strength value measured after annealing.

EXAMPLE 1

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 1 Chemical components (wt %) No. C Cu S Al N P B Nb Others A11 0.0008 0.15 0.008 0.029 0.0014 0.048 0.0005 0.009 — A12 0.0012 0.17 0.007 0.024 0.0014 0.052 0.0002 0.011 Si: 0.02 A13 0.0021 0.09 0.018 0.044 0.0019 0.083 0.0007 0.027 Si: 0.15 A14 0.0031 0.12 0.011 0.028 0.0024 0.118 0.0011 0.024 Si: 0.25 A15 0.0029 0.08 0.009 0.038 0.0018 0.085 0.0008 0.021 Si: 0.17 Mo: 0.08 A16 0.0023 0.11 0.011 0.039 0.0029 0.088 0.001 0.032 Si: 0.18 Cr: 0.19 A17 0.0024 0.11 0.01 0.035 0.0018 0.053 0 0 A18 0.0044 0 0.008 0.024 0.0021 0.122 0.0007 0.077

TABLE 2 (Cu/ 63.5)/ Av. size of CuS Number of CuS No. (S/32) (Nb/93)/(C/12) precipitates (μm) precipitates (/mm²) A11 9.45 1.45 0.05 2.2 × 10⁷ A12 12.2 1.18 0.06 1.2 × 10⁷ A13 2.52 1.66 0.05 3.2 × 10⁷ A14 5.5 1 0.05 3.2 × 10⁷ A15 4.48 0.93 0.05 4.1 × 10⁷ A16 5.04 1.8 0.05 5.3 × 10⁷ A17 5.54 0 0.06 5.5 × 10⁶ A18 0 2.26 0.05 5.4 × 10³

TABLE 3 Mechanical properties AI SWE No. YS (MPa) TS (MPa) El (%) r_(m) Δr (%) (DBTT-° C.) Remarks A11 216 347 46 2.01 0.25 0 −70 IS A12 224 354 44 1.97 0.24 0 −70 IS A13 262 409 38 1.78 0.22 0 −60 IS A14 327 464 35 1.61 0.21 0 −50 IS A15 321 457 34 1.72 0.23 0 −50 IS A16 335 462 34 1.69 0.21 0 −60 IS A17 239 348 42 1.18 0.29 0.62 −70 CS A18 325 465 25 1.49 0.48 0 −50 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 2

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 4 Chemical components (wt %) No. C Mn Cu S Al N P B Nb Others A21 0.0015 0.07 0.09 0.011 0.042 0.0024 0.041 0.001 0.032 A22 0.0016 0.05 0.05 0.015 0.04 0.0018 0.04 0.0007 0.011 Si: 0.04 A23 0.0028 0.08 0.08 0.015 0.03 0.0023 0.086 0.0007 0.031 Si: 0.24 A24 0.0018 0.07 0.12 0.007 0.05 0.0019 0.125 0.005 0.035 Si: 0.35 A25 0.0019 0.09 0.09 0.011 0.048 0.0023 0.08 0.0009 0.027 Si: 0.23 Mo: 0.09 A26 0.0029 0.11 0.1 0.009 0.039 0.0031 0.075 0.001 0.031 Si: 0.31 Cr: 0.24 A27 0.0038 0.42 0 0.0083 0.038 0.0024 0.052 0.005 0.051 A28 0.0015 0.07 0.08 0.012 0.032 0.0021 0.118 0 0 Si: 0.1

TABLE 5 (Mn/55 + Cu/63.5)/ (Nb/93)/(C/ Av. size of CuS Number of CuS No. Mn + Cu (S/32) 12) precipitates (μm) precipitates (/mm²) A21 0.16 7.27 2.77 0.05 6.2 × 10⁷ A22 0.1 3.62 0.89 0.04 5.5 × 10⁷ A23 0.16 5.79 1.43 0.04 6.0 × 10⁷ A24 0.19 14.5 2.51 0.03 7.2 × 10⁷ A25 0.18 8.88 1.83 0.04 6.5 × 10⁷ A26 0.21 12.7 1.38 0.04 6.9 × 10⁷ A27 0.42 29.4 1.73 0.25 1.5 × 10⁴ A28 0.15 6.75 0 0.06 5.3 × 10⁶

TABLE 6 Mechanical properties El SWE No. YS (MPa) TS (MPa) (%) r_(m) Δr AI (%) (DBTT-° C.) Remarks A1 226 362 44 2.03 0.22 0 −70 IS A2 225 348 45 2.12 0.19 0 −70 IS A3 282 402 39 1.87 0.21 0 −60 IS A4 338 451 34 1.68 0.16 0 −50 IS A5 329 449 34 1.88 0.22 0 −50 IS A6 383 452 35 1.64 0.19 0 −50 IS A7 188 342 42 1.77 0.39 0 −60 CS A8 378 463 30 1.25 0.28 0.42 −50 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 3

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 7 Chemical components (wt %) No. C Cu S Al N P B Nb Others A31 0.0019 0.17 0.009 0.024 0.0054 0.052 0.0007 0.028 A32 0.0015 0.06 0.009 0.044 0.0077 0.045 0.0004 0.031 Si: 0.03 A33 0.0024 0.11 0.01 0.068 0.008 0.085 0.0006 0.037 Si: 0.11 A34 0.0024 0.17 0.011 0.058 0.014 0.126 0.0004 0.025 Si: 0.18 A35 0.0028 0.12 0.009 0.043 0.0093 0.044 0.0005 0.032 Si: 0.07 Mo: 0.06 A36 0.0025 0.09 0.012 0.033 0.012 0.039 0.0009 0.021 Cr: 0.22 A37 0.0036 0.35 0.01 0.034 0.0012 0.042 0.0005 0.074 A38 0.0014 0.42 0.009 0.055 0.0067 0.12 0.0005 0 Si: 0.13

TABLE 8 (Cu/ Av. size of Number of CuS 63.5)/ (Nb/93)/ (Al/27)/ CuS precipitates precipitates No. (S/32) (C/12) (N/14) (μm) (/mm²) A31 9.44 1.92 2.31 0.04 6.4 × 10⁷ A32 3.36 2.67 2.96 0.04 7.5 × 10⁷ A33 5.54 1.99 4.41 0.04 7.0 × 10⁷ A34 7.79 1.34 2.15 0.03 6.2 × 10⁷ A35 6.72 1.47 2.4 0.04 7.5 × 10⁷ A36 3.78 1.08 1.43 0.04 7.9 × 10⁷ A37 17.6 2.65 14.7 0.25 4.5 × 10⁴ A38 23.5 0 4.26 0.06 4.3 × 10⁵

TABLE 9 Mechanical properties SWE YS TS El AI (DBTT- Re- No. (MPa) (MPa) (%) r_(m) Δr (%) ° C.) marks A31 227 353 44 1.92 0.19 0 −70 A32 209 352 42 2.07 0.24 0 −40 IS A33 261 402 37 1.88 0.27 0 −40 IS A34 341 452 33 1.88 0.22 0 −40 IS A35 231 392 37 1.94 0.23 0 −50 IS A36 226 372 38 1.74 0.21 0 −40 IS A37 242 369 36 1.62 0.42 0.62 −60 CS A38 373 465 33 1.21 0.57 0 −40 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 4

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 10 Chemical Components (wt %) No. C Mn Cu S Al N P B Nb Others A41 0.0018 0.12 0.08 0.012 0.044 0.0069 0.036 0.0007 0.015 Si: 0.2 A42 0.0018 0.07 0.06 0.015 0.05 0.0063 0.04 0.0007 0.01 Si: 0.05 A43 0.0022 0.09 0.09 0.013 0.05 0.0082 0.083 0.0007 0.031 Si: 0.11 A44 0.0025 0.07 0.12 0.009 0.04 0.0078 0.129 0.005 0.028 Si: 0.15 A45 0.0026 0.11 0.11 0.011 0.041 0.012 0.029 0.0008 0.032 Si: 0.16 Mo: 0.07 A46 0.0032 0.09 0.09 0.012 0.036 0.0093 0.031 0.0011 0.028 Si: 0.15 Cr: 0.25 A47 0.0034 0.45 0 0.0083 0.038 0.0015 0.048 0.005 0.063 A48 0.0038 0.07 0.08 0.012 0.035 0.0024 0.13 0.005 0

TABLE 11 Av. size of Number of (Mn/55 + (Nb/ CuS CuS Mn + Cu/63.5)/ 93)/ (Al/27)/ precipitates precipitates No. Cu (S/32) (C/12) (N/14) (μm) (/mm*) A41 0.2 8.33 1.08 3.32 0.04 6.9 × 10⁷ A42 0.13 4.73 0.72 4.12 0.04 8.4 × 10⁷ A43 0.18 7.52 1.82 3.16 0.04 9.0 × 10⁷ A44 0.19 11.2 1.45 2.66 0.04 8.2 × 10⁷ A45 0.22 10.9 1.59 1.77 0.04 6.9 × 10⁷ A46 0.18 8.14 1.13 2.01 0.03 9.4 × 10⁷ A47 0.45 31.5 2.39 13.1 0.25 1.5 × 10⁴ A48 0.15 6.75 0 7.56 0.04 3.5 × 10⁵

TABLE 12 Mechanical properties SWE YS TS El AI (DBTT- Re- No. (MPa) (MPa) (%) r_(m) Δr (%) ° C.) marks A41 238 368 44 2.18 0.22 0 −70 IS A42 220 345 44 2.25 0.19 0 −70 IS A43 268 403 38 1.82 0.17 0 −60 IS A44 325 461 34 1.79 0.19 0 −60 IS A45 234 359 42 2.32 0.23 0 −60 IS A46 228 355 43 2.25 0.25 0 −50 IS A47 202 355 38 1.59 0.39 0 −60 CS A48 338 458 24 1.31 0.58 0.55 −70 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 5

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 13 Chemical components (wt %) No. C Mn P S Al Nb B N Others A51 0.0015 0.05 0.05 0.012 0.04 0.021 0.0007 0.0018 A52 0.0028 0.08 0.082 0.015 0.041 0.019 0.0007 0.0027 Si: 0.15 A53 0.0038 0.12 0.12 0.006 0.028 0.035 0.0011 0.0018 A54 0.0019 0.07 0.09 0.011 0.032 0.021 0.0009 0.0021 Mo: 0.06 A55 0.0027 0.05 0.09 0.009 0.041 0.033 0.001 0.0029 Cr: 0.13 A56 0.0024 0.07 0.053 0.01 0.035 0 0 0.0012 A57 0.0024 0.32 0.11 0.008 0.024 0.035 0.007 0.0013

TABLE 14 (Mn/55)/ (Nb/93)/ Av. size of CuS Number of CuS No. (S/32) (C/12) precipitates (μm) precipitates (/mm²) A51 2.42 1.81 0.06 4.2 × 10⁵ A52 3.1  0.88 0.05 5.2 × 10⁵ A53 11.6  1.19 0.05 3.2 × 10⁶ A54 3.7  1.43 0.05 2.9 × 10⁶ A55 3.23 1.58 0.04 3.2 × 10⁶ A56 4.07 0   0.06 4.5 × 10⁴ A57 23.3  1.88 0.22 2.3 × 10³

TABLE 15 Mechanical properties SWE YS TS El AI (DBTT- Re- No. (MPa) (MPa) (%) r_(m) Δr (%) ° C.) marks A51 215 358 45 1.92 0.25 0 −70 IS A52 254 403 39 1.73 0.22 0 −60 IS A53 315 458 35 1.59 0.19 0 −50 IS A54 288 442 36 1.78 0.24 0 −40 IS A55 309 452 35 1.52 0.21 0 −50 IS A56 248 355 41 1.33 0.29 0 −40 CS A57 254 454 25 1.56 0.28 0 −70 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 6

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 16 Chemical Component (wt %) No. C S Al N P B Nb Others A61 0.0014 0.006 0.053 0.0072 0.051 0.0005 0.015 A62 0.0023 0.01 0.062 0.0082 0.082 0.0004 0.021 Si: 0.12 A63 0.0018 0.011 0.055 0.0127 0.118 0.0004 0.037 Si: 0.25 A64 0.0024 0.008 0.049 0.0074 0.029 0.0007 0.028 Si: 0.2 Mo: 0.06 A65 0.0032 0.013 0.053 0.0088 0.109 0.0009 0.035 Si: 0.18 Cr: 0.13 A66 0.0018 0.013 0.052 0.0018 0.052 0.005 0 A67 0.0035 0.009 0.008 0.023 0.125 0.0005 0.062

TABLE 17 (Al/27)/ (Nb/93)/(C/ Av. size of CuS Number of CuS No. (N/14) 14) precipitates (μm) Precipitates (/mm²) A61 3.82 1.38 0.06 5.3 × 10⁵ A62 3.92 1.18 0.05 5.6 × 10⁵ A63 2.25 2.65 0.05 6.8 × 10⁶ A64 3.43 1.51 0.05 5.5 × 10⁶ A65 3.12 1.41 0.04 6.3 × 10⁶ A66 15    0   0.11 4.5 × 10⁴ A67 0.18 2.29 0.08 8.4 × 10⁴

TABLE 18 Mechanical properties SWE YS TS EL (DBTT- AI No. (MP) (MPa) (%) r_(m) Δr ° C.) (%) Remarks A61 209 349 44 2.03 0.25 −60 0 IS A62 282 399 37 1.72 0.24 −50 0 IS A63 339 457 34 1.73 0.27 −50 0 IS A64 219 360 42 2.21 0.29 −50 0 IS A65 354 449 33 1.73 0.21 −60 0 IS A66 189 348 45 1.32 0.43 −40 0.94 CS A67 335 457 26 1.53 0.24 −40 0 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 7

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 19 Chemical component (wt %) No. C Mn P S Al Nb B N Others A71 0.0013 0.11 0.041 0.006 0.054 0.014 0.0005 0.0072 A72 0.0026 0.1 0.075 0.01 0.072 0.018 0.0007 0.0082 Si: 0.11 A73 0.0018 0.09 0.105 0.011 0.055 0.037 0.0005 0.0127 Si: 0.1 A74 0.0031 0.07 0.035 0.009 0.043 0.032 0.0005 0.0079 Si: 0.12 Mo: 0.06 A75 0.0021 0.14 0.036 0.01 0.052 0.019 0.0007 0.0089 Cr: 0.13 A76 0.0018 0.68 0.045 0.009 0.048 0.022 0.0004 0.0021 A77 0.0037 0.1 0.114 0.01 0.008 0.01 0.0011 0.0067 Si: 0.04

TABLE 20 (Nb/ Av. size of CuS Number of CuS (Mn/55)/ (Al/27)/ 93)/ precipitates precipitates No. (S/32) (N/14) (C/12) (μm) (/mm²) A71 10.7 3.89 1.39 0.06 5.2 × 10⁵ A72 5.82 4.55 0.89 0.05 6.4 × 10⁵ A73 4.76 2.25 2.65 0.05 7.5 × 10⁶ A74 4.53 2.82 1.33 0.05 6.8 × 10⁶ A75 8.15 3.03 1.17 0.05 7.3 × 10⁶ A76 44 11.9  1.58 0.24 1.8 × 10³ A77 5.82 0.62 0.35 0.06 4.5 × 10⁴

TABLE 21 Mechanical properties SWE YS TS EL (DBTT- AI Re- No. (MPa) (MPa) (%) r_(m) Δr ° C.) (%) marks A71 205 357 45 1.99 0.25 −60 0 IS A72 275 398 38 1.82 0.29 −50 0 IS A73 345 453 34 1.83 0.27 −60 0 IS A74 232 363 42 2.12 0.24 −50 0 IS A75 229 362 44 1.89 0.22 −50 0 IS A76 185 348 42 1.92 0.42 −40 0 CS A77 378 461 27 1.12 0.34 −60 0.49 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 8

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 22 Chemical component (wt %) No. C P S Al Cu Nb B N Others B81 0.0028 0.008 0.008 0.029 0.04 0.009 0.0005 0.0014 B82 0.0032 0.077 0.01 0.035 0.09 0.004 0.0005 0.0019 B83 0.003 0.048 0.009 0.034 0.12 0.006 0.0005 0.0017 Si: 0.03 B84 0.0019 0.081 0.015 0.04 0.11  0.0004 0.0007 0.0022 Si: 0.14 B85 0.0044 0.112 0.013 0.023 0.09 0.012 0.0012 0.0013 Si: 0.26 B86 0.0028 0.082 0.011 0.033 0.16 0.006 0.0006 0.0018 Si: 0.15 Mo: 0.084 B87 0.0037 0.085 0.01 0.025 0.12 0.006 0.001  0.0022 Si: 0.15 Cr: 0.15 B88 0.0028 0.05 0.013 0.038 0.13 0.055 0 0.0014 — B89 0.0038 0.119 0.012 0.029 0    0    0.0005 0.0026 —

TABLE 23 (Cu/63.5)/ Av. size of CuS Number of CuS No. (S/32) Cs precipitates (μm) precipitates (/mm²) B81 2.52 16.387 0.05 4.7 × 10⁶ B82 4.54 26.839 0.05 6.6 × 10⁶ B83 6.72 22.258 0.06 6.8 × 10⁶ B84 3.7  18.484 0.05 9.12 × 10⁶  B85 3.49 28.516 0.05 3.4 × 10⁷ B86 7.33 20.258 0.05 1.1 × 10⁷ B87 6.05 29.258 0.05 2.1 × 10⁷ B88 5.04 −42.97 0.05 2.7 × 10⁷ B89 0   38 0.07 5.4 × 10⁶ Cs = (C − Nb × 12/93) × 10000

TABLE 24 Mechanical properties YS TS EL BH value SWE No. (Mpa) (MPa) (%) r_(m) Δr AI (%) (MPa) (DBTT-° C.) Remarks B81 193 302 49 1.93 0.23 0 44 −50 B82 241 372 42 1.71 0.28 0 59 −50 B83 234 356 44 1.66 0.25 0 42 −60 IS B84 271 404 39 1.52 0.32 0 53 −70 IS B85 331 467 34 1.31 0.18 0 62 −60 IS B86 329 452 35 1.73 0.24 0 44 −50 IS B87 343 460 34 1.67 0.21 0 57 −60 IS B88 209 345 40 1.88 0.29 0 0 −10 CS B89 328 469 29 1.19 0.19 2.8 95 −70 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 9

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 25 Chemical component (wt %) No. C Mn P S Al Cu Nb B N Others B91 0.0018 0.1 0.008 0.009 0.035 0.082 0.003 0.0007 0.0013 — B92 0.0022 0.12 0.026 0.011 0.042 0.1 0.004 0.0005 0.0022 — B93 0.0018 0.07 0.044 0.012 0.029 0.068 0.003 0.0008 0.0027 Si: 0.05 B94 0.0028 0.11 0.082 0.013 0.043 0.084 0.006 0.0006 0.0023 Si: 0.26 B95 0.0039 0.07 0.12 0.011 0.035 0.11 0.008 0.0008 0.0023 Si: 0.39 B96 0.0028 0.09 0.083 0.012 0.033 0.15 0.006 0.0005 0.0021 Si: 0.27 Mo: 0.085 B97 0.0037 0.08 0.073 0.01 0.052 0.13 0.008 0.0009 0.0015 Si: 0.33 Cr: 0.28 B98 0.0025 0.42 0.055 0.009 0.043 0 0.038 0.005 0.0024 — B99 0.0039 0.07 0.115 0.011 0.036 0.11 0.003 0 0.0027 Si: 0.1

TABLE 26 (Mn/55 + Av. size of CuS Number of CuS Cu + Cu/63.5)/ precipitates precipitates No. Mn (S/32) Cs (μm) (/mm²) B91 0.18 11.1 14.129 0.06 1.1 × 10⁷ B92 0.22 10.9 16.839 0.06 1.8 × 10⁷ B93 0.14 6.25 14.129 0.06 1.5 × 10⁷ B94 0.19 8.18 20.258 0.05 2.5 × 10⁷ B95 0.18 8.74 28.677 0.05 3.2 × 10⁷ B96 0.24 10.7 20.258 0.05 2.5 × 10⁷ B97 0.21 11.2 26.677 0.04 4.4 × 10⁷ B98 0.42 27.2 −24.03 0.25 6.5 × 10⁴ B99 0.18 8.74 35.129 0.06 5.3 × 10⁶ Cs = (C − Nb × 12/93) × 10000

TABLE 27 Mechanical properties YS TS EL BH value SWE No. (MPa) (MPa) (%) r_(m) Δr AI (%) (MPa) (DBTT-° C.) Remarks B91 190 308 48 1.91 0.31 0 35 −50 IS B92 209 329 46 1.85 0.27 0 41 −50 IS B93 227 347 46 1.82 0.22 0 36 −70 IS B94 295 396 39 1.73 0.21 0 45 −60 IS B95 341 463 33 1.53 0.14 0 58 −50 IS B96 331 446 35 1.71 0.22 0 51 −50 IS B97 355 457 34 1.59 0.18 0 46 −50 IS B98 196 350 40 1.85 0.29 0 0 −50 CS B99 369 451 32 1.29 0.198 2.5 95 −30 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 10

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 28 Chemical component (wt %) No. C P S Al Cu Nb B N Others B01 0.0021 0.009 0.01 0.042 0.08 0.005 0.0007 0.0072 B02 0.0022 0.027 0.009 0.039 0.12 0.004 0.0005 0.0092 B03 0.0018 0.046 0.012 0.046 0.07 0.003 0.0004 0.0086 Si: 0.11 B04 0.0039 0.082 0.009 0.052 0.12 0.008 0.0009 0.0092 Si: 0.08 B05 0.004 0.122 0.011 0.039 0.15 0.009 0.0006 0.0121 Si: 0.21 B06 0.0028 0.045 0.011 0.049 0.09 0.004 0.0005 0.0085 Si: 0.07 Mo: 0.064 B07 0.0039 0.041 0.011 0.042 0.12 0.008 0.0009 0.011 Cr: 0.22 B08 0.0028 0.043 0.012 0.036 0.37 0.053 0.0009 0.0022 B09 0.0042 0.116 0.011 0.052 0.35 0 0.0005 0.0021 Si: 0.13

TABLE 29 Av. size of CuS Number of CuS (Cu/63.5)/ (Al/27)/ precipitates precipitates No. (S/32) (N/14) Cs (μm) (/mm²) B01 4.03 3.02 14.548 0.04 5.8 × 10⁷ B02 6.72 2.2 16.839 0.04 6.9 × 10⁷ B03 2.94 2.77 14.129 0.04 6.5 × 10⁷ B04 6.72 2.93 28.677 0.04 7.8 × 10⁷ B05 6.87 1.67 28.387 0.04 6.8 × 10⁷ B06 4.12 2.99 22.839 0.04 5.5 × 10⁷ B07 5.5 1.98 28.677 0.04 6.9 × 10⁷ B08 15.5 8.48 −40.39 0.25 6.5 × 10⁴ B09 16 12.8 42 0.06 5.5 × 10⁵ Cs = (C − Nb × 12/93) × 10000

TABLE 30 Mechanical properties YA Ts EL BH value SWE No. (MPa) (MPa) (%) r_(m) Δr AI (%) (MPa) (DBTT-° C.) Remarks B01 203 318 47 1.92 0.28 0 36 −50 IS B02 218 345 44 1.82 0.27 0 42 −40 IS B03 209 352 42 1.85 0.24 0 42 −40 IS B04 261 402 37 1.68 0.27 0 53 −50 IS B05 337 455 33 1.53 0.22 0 56 −40 IS B06 237 390 38 1.64 0.23 0 63 −50 IS B07 228 378 40 1.71 0.21 0 55 −40 IS B08 238 366 38 1.72 0.38 0 0 −60 CS B09 373 465 33 1.21 0.57 3.8 95 −40 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 11

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 31 Chemical Component (wt %) No. C Mn P S Al Cu Nb B N Others B11 0.0019 0.08 0.008 0.009 0.042 0.11 0.003 0.0006 0.0074 B12 0.0022 0.1 0.027 0.009 0.039 0.09 0.005 0.0005 0.0092 B13 0.0016 0.11 0.042 0.013 0.052 0.08 0.003 0.0005 0.0073 Si: 0.09 B14 0.0027 0.09 0.085 0.011 0.045 0.11 0.006 0.0007 0.0082 Si: 0.1 B15 0.0039 0.12 0.12 0.012 0.024 0.09 0.008 0.0005 0.0084 Si: 0.12 B16 0.0032 0.09 0.033 0.009 0.047 0.11 0.005 0.0008 0.011 Si: 0.15 Mo: 0.081 B17 0.0038 0.09 0.035 0.01 0.033 0.13 0.008 0.0011 0.00105 Si: 0.17Cr: 0.27 B18 0.0033 0.47 0.045 0.0053 0.039 0 0.033 0.0006 0.0015 B19 0.0045 0.18 0.128 0.008 0.045 0.12 0.006 0.0008 0.0018 Si: 0.11

TABLE 32 (Mn/55 + Cu/ Av. size of CuS Number of CuS 63.5)/(S/ (Al/27)/ precipitates precipitates No. Cu + Mn 32) (N/14) Cs (μm) (/mm²) B11 0.19 11.3 2.94 15.129 0.05 2.2 × 10⁷ B12 0.19 11.5 2.2 15.548 0.05 2.7 × 10⁷ B13 0.19 8.02 3.69 12.129 0.05 2.1 × 10⁷ B14 0.2 9.8 2.85 19.258 0.05 3.1 × 10⁷ B15 0.21 9.6 1.48 28.677 0.04 4.5 × 10⁷ B16 0.2 12 2.22 25.548 0.05 3.4 × 10⁷ B17 0.22 11.8 16.3 27.677 0.05 2.7 × 10⁷ B18 0.47 51.6 13.5 −9.581 0.25 2.9 × 10⁴ B19 0.3 20.6 13 37.258 0.06 5.5 × 10⁵ Cs = (C − Nb × 12/93) × 10000

TABLE 33 Mechanical properties YS TS EL BH value SWE No. (MPa) (MPa) (%) r_(m) Δr AI (%) (MPa) (DBTT-° C.) Remarks B11 196 322 49 1.92 0.31 0 37 −50 IS B12 208 342 46 1.87 0.29 0 42 −50 IS B13 227 352 43 1.85 0.28 0 34 −60 IS B14 263 397 39 1.7 0.25 0 48 −50 IS B15 336 449 33 1.58 0.22 0 62 −40 IS B16 240 355 43 1.88 0.33 0 50 −60 IS B17 232 361 42 1.75 0.31 0 66 −50 IS B18 209 348 39 1.95 0.36 0 0 −50 CS B19 342 461 29 1.33 0.21 3.5 106 −60 CS *Note: YS = Yield strength, TS = Tensile strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 12

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 34 Chemical component (wt %) No. C Mn P S Al Nb B N Others B21 0.0021 0.12 0.008 0.009 0.034 0.004 0.0008 0.0023 B22 0.0018 0.07 0.045 0.013 0.047 0.003 0.0005 0.0021 Si: 0.12 B23 0.0031 0.11 0.085 0.012 0.039 0.006 0.0008 0.0025 B24 0.004 0.09 0.123 0.01 0.034 0.008 0.0009 0.0011 B25 0.004 0.09 0.092 0.009 0.041 0.009 0.0005 0.0027 Mo: 0.065 B26 0.0033 0.05 0.094 0.011 0.039 0.007 0.0008 0.0018 Cr: 0.16 B27 0.0027 0.48 0.051 0.009 0.028 0.032 0.0007 0.0022 B28 0.0042 0.09 0.11 0.011 0.028 0 0.007 0.0016

TABLE 35 (Mn/55)/(S/ Av. size of CuS Number of CuS No. 32) Cs precipitates (μm) precipitates (/mm²) B21 7.76 15.839 0.06 1.9 × 10⁵ B22 3.13 14.129 0.06 2.9 × 10⁵ B23 5.33 23.258 0.05 3.7 × 10⁵ B24 5.24 29.677 0.05 4.4 × 10⁶ B25 5.82 28.387 0.05 3.2 × 10⁶ B26 2.64 23.968 0.05 2.9 × 10⁶ B27 31 −14.29 0.06 3.2 × 10⁴ B28 4.76 42 0.22 2.3 × 10⁵ Cs = (C − Nb × 12/93) × 10000

TABLE 36 Mechanical properties YS TS EL BH value SWE No. (MPa) (MPa) (%) r_(m) Δr AI (%) (MPa) (DBTT-° C.) Remarks B21 188 302 51 1.98 0.36 0 39 −40 IS B22 210 347 46 1.81 0.31 0 38 −50 IS B23 259 409 38 1.63 0.24 0 53 −60 IS B24 325 451 35 1.55 0.19 0 66 −50 IS B25 293 449 36 1.51 0.2 0 58 −40 IS B26 302 452 36 1.45 0.18 0 57 −50 IS B27 245 352 40 1.83 0.29 0 0 −40 CS B28 254 454 25 1.56 0.28 3.8 92 −60 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 13

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 37 Chemical component (wt %) No. C P S Al Nb B N Others B31 0.0025 0.009 0.01 0.028 0.005 0.0008 0.0071 B32 0.0019 0.011 0.009 0.034 0.004 0.0007 0.0093 Si: 0.24 B33 0.0018 0.054 0.011 0.032 0.002 0.0005 0.0072 Si: 0.07 B34 0.0035 0.084 0.009 0.045 0.008 0.0008 0.0066 Si: 0.11 B35 0.004 0.11 0.012 0.062 0.009 0.0004 0.0091 Si: 0.22 B36 0.0039 0.033 0.009 0.042 0.007 0.0007 0.0071 Si: 0.2 Mo: 0.062 B37 0.0035 0.11 0.009 0.044 0.008 0.0009 0.012 Si: 0.19 Cr: 0.15 B38 0.0025 0.05 0.008 0.052 0.03 0.0005 0.0018 B39 0.0036 0.12 0.011 0.042 0 0.0005 0.013

TABLE 38 Av. size of CuS Number of CuS (Al/27)/(N/ precipitates precipitates No. 14) Cs (μm) (/mm²) B31 2.04 18.548 0.06 2.1 × 10⁵ B32 1.9 13.839 0.06 2.3 × 10⁵ B33 2.3 15.419 0.06 2.5 × 10⁵ B34 3.54 24.677 0.05 3.2 × 10⁵ B35 3.53 28.387 0.05 4.2 × 10⁶ B36 3.07 29.968 0.05 3.1 × 10⁵ B37 1.9 24.677 0.05 4.5 × 10⁵ B38 15 −13.71 0.32 1.6 × 10⁴ B39 1.68 36 0.06 2.8 × 10⁵ Cs = (C − Nb× 12/93) × 10000

TABLE 39 Mechanical properties YS TS EL AI BH value SWE No. (MPa) (MPa) (%) r_(m) Δr (%) (MPa) (DBTT-° C.) Remarks B31 197 315 49 1.93 0.37 0 43 −40 IS B32 208 342 45 1.92 0.36 0 39 −40 IS B33 209 349 44 1.85 0.31 0 33 −40 IS B34 275 390 37 1.7 0.24 0 62 −50 IS B35 335 452 33 1.52 0.22 0 72 −40 IS B36 224 362 40 1.72 0.29 0 53 −60 IS B37 371 452 33 1.55 0.18 0 54 −60 IS B38 197 332 45 1.93 0.43 0 0 −40 CS B39 355 457 31 1.42 0.24 0 −40 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

EXAMPLE 14

First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400° C./min., wound at 650° C., cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910° C., which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10° C./second to 830° C. for 40 seconds to produce the final cold rolled steel sheets.

TABLE 40 Chemical component (wt %) No. C Mn P S Al Nb B N Others B41 0.0028 0.08 0.009 0.009 0.039 0.006 0.0005 0.0092 B42 0.0022 0.1 0.026 0.01 0.048 0.005 0.0007 0.0072 B43 0.0019 0.09 0.043 0.011 0.042 0.004 0.0005 0.0082 Si: 0.04 B44 0.0033 0.12 0.078 0.009 0.067 0.005 0.0008 0.0078 Si: 0.11 B45 0.0039 0.09 0.097 0.015 0.037 0.008 0.001 0.0105 Si: 0.08 B46 0.0035 0.11 0.032 0.01 0.028 0.008 0.0007 0.0059 Si: 0.11 Mo: 0.058 B47 0.0029 0.07 0.041 0.008 0.033 0.006 0.0007 0.008 Si: 0.05 Cr: 0.33 B48 0.0027 0.68 0.041 0.008 0.035 0.028 0.0008 0.002 Si: 0.08 B49 0.0042 0.08 0.114 0.01 0.008 0 0.0011 0.0067 Si: 0.05

TABLE 41 Av. size of CuS Number of CuS (Mn/55)/ (Al/27)/ precipitates precipitates No. (S/32) (N/14) Cs (μm) (/mm²) B41 5.17 2.2 20.258 0.05 4.2 × 10⁵ B42 5.82 3.46 15.548 0.05 3.5 × 10⁵ B43 4.76 2.66 13.839 0.05 3.8 × 10⁵ B44 7.76 4.45 26.548 0.05 5.8 × 10⁵ B45 3.49 1.83 28.677 0.05 5.4 × 10⁶ B46 6.4 2.46 24.677 0.05 4.5 × 10⁶ B47 5.09 2.14 21.258 0.05 3.7 × 10⁶ B48 49.5 9.07 −9.129 0.32 1.2 × 10⁴ B49 4.65 0.62 42 0.06 2.8 × 10⁵ Cs = (C − Nb × 12/93) × 10000

TABLE 42 Mechanical Properties YS TS EL AI BH value SWE No. (MPa) (MPa) (%) r_(m) Δr (%) (MPa) (DBTT-° C.) Remarks B41 197 320 49 1.93 0.32 0 59 −40 IS B42 209 342 46 1.84 0.29 0 44 −50 IS B43 214 355 43 1.82 0.29 0 35 −50 IS B44 275 398 38 1.71 0.25 0 69 −50 IS B45 348 452 32 1.55 0.18 0 74 −60 IS B46 242 359 40 1.79 0.24 0 59 −50 IS B47 225 360 42 1.75 0.22 0 48 −40 IS B48 197 359 40 1.92 0.37 0 0 −50 CS B49 378 461 27 1.12 0.34 5.2 105 −60 CS *Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, r_(m) = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

The preferred embodiments illustrated in the present invention do not serve to limit the present invention, but are set forth for illustrative purposes. Any embodiment having substantially the same constitution and the same operational effects thereof as the technical spirit of the present invention as defined in the appended claims is encompassed within the technical scope of the present invention.

As apparent from the above description, according to the cold rolled steel sheets of the present invention, the distribution of fine precipitates in Nb based IF steels allows the formation of minute crystal grains, and as a result, the in-plane anisotropy index is lowered and the yield strength is enhanced by precipitation enhancement. 

1. A cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled steel sheet having a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies the following relationship: 1≦(Cu/63.5)/(S/32)≦30, and wherein the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.
 2. The cold rolled steel sheet according to claim 1, wherein the composition further comprises 0.01 to 0.3% of Mn, and satisfies the following relationship: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.
 3. The cold rolled steel sheet according to claim 1, wherein the N content is 0.004 to 0.02%, and the composition satisfies the following relationship: 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.
 4. The cold rolled steel sheet according to claim 1, wherein the composition further comprising 0.01 to 0.3% of Mn and 0.004 to 0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.
 5. A cold rolled steel sheet with high yield ratio and low in-plane anisotropy index, the cold rolled sheet having a composition comprising: 0.01% or less C, 0.08% or less S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.0001 to 0.002% of B, 0.002 to 0.04% of Nb, at least one kind selected from 0.01 to 0.2% of Cu, 0.01 to 0.3% of Mn and 0.004 to 0.2% of N, by weight, and the balance Fe and other unavoidable impurities, wherein the composition satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, where the N content is 0.004% or more, and wherein the steel sheet comprises at least one kind selected from (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.
 6. The cold rolled steel sheet according to claim 1 or 5, wherein the C and Nb contents satisfy the following relationship, by weight: 0.8≦(Nb/93)/(C/12)≦5.0.
 7. The cold rolled steel sheet according to claim 6, wherein the C content is 0.005% or less.
 8. The cold rolled steel sheet according to claim 1 or 5, wherein solute carbon (Cs) is from 5 to 30, where Cs=(C−Nb×12/93)×10,000.
 9. The cold rolled steel sheet according to claim 8, wherein the C content is from 0.001 to 0.01%.
 10. The cold rolled steel sheet according to one of preceding claims 1 to 5, wherein the cold rolled steel satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
 11. The cold rolled steel sheet according to one of preceding claims 1 to 5, wherein the number of the precipitates is 1×10⁶/mm² or more.
 12. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is 0.015% or less.
 13. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is from 0.03 to 0.2%.
 14. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises at least one kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.
 15. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises 0.01 to 0.2% of Mo.
 16. The cold rolled steel sheet according to claim 14, wherein the composition further comprises 0.01 to 0.2% of Mo.
 17. The cold rolled steel sheet according to claim 2, 4 or 5, wherein the sum of Mn and Cu is from 0.08 to 0.4%.
 18. The cold rolled steel sheet according to claim 2, 4 or 5, wherein the Mn content is from 0.01 to 0.12%.
 19. The cold rolled steel sheet according to claim 2, 4 or 5, wherein the value of (Mn/55+Cu/63.5)/(S/32) is from 1 to
 9. 20. The cold rolled steel sheet according to claim 3, 4 or 5, wherein the value of (Al/27)/(N/14) is from 1 to
 5. 21. A method of producing a cold roller steel sheet with high yield ratio and low in-plane anisotropy index, the method comprising steps of: reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less C, 0.01 to 0.2% of Cu, 0.005 to 0.08% of S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, by weight, and the balance Fe and other unavoidable impurities, the composition satisfying the following relationship: 1≦(Cu/63.5)/(S/32)≦30; hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet; cooling the hot rolled steel sheet at a rate of 300° C./min or higher; winding the cooled steel sheet at 700° C. or lower; cold rolling the wound steel sheet; and continuously annealing the cold rolled steel sheet, the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.
 22. The method according to claim 21, wherein the composition further comprising 0.01 to 0.3% of Mn, and satisfies the following relationship: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 μm or less.
 23. The method according to claim 21, wherein the N content is 0.004 to 0.02%, and the composition satisfies the following relationship: 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.
 24. The method according to claim 21, wherein the composition further comprises 0.01 to 0.3% of Mn and 0.004 to 0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)/(N/14)≦10, and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.
 25. A method of producing a cold roller steel sheet with high yield ratio and low in-plane anisotropy index, the method comprising steps of: reheating a slab to a temperature of 1,100° C. or higher, the slab having a composition comprising 0.01% or less C, 0.08% of less S, 0.1% or less Al, 0.004% or less N, 0.2% or less P, 0.001 to 0.002% of B, 0.002 to 0.04% Nb, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, by weight, and the balance Fe and other unavoidable impurities, the composition satisfying the following relationships: 1≦(Mn/55+Cu/63.5)/(S/32)≦30, 1≦(Al/27)(N/14)≦10, where the N content is 0.004% or more,; hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet; cooling the hot rolled steel sheet at a rate of 300° C./min or higher; winding the cooled steel sheet at 700° C. or lower; cold rolling the wound steel sheet; and continuously annealing the cold rolled steel sheet, the steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.
 26. The method according to claim 21 or 25, wherein the C and Nb contents satisfy the following relationship, by weight: 0.8≦(Nb/93)/(C/12)≦5.0.
 27. The method according to claim 26, wherein the C content is 0.005% or less.
 28. The method according to claim 21 or 25, wherein solute carbon (Cs) is from 5 to 30, where Cs=(C−Nb×12/93)×10,000.
 29. The method according to claim 28, wherein the C content is from 0.001 to 0.01%.
 30. The method according to one of preceding claims 21 to 25, wherein the cold rolled steel satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
 31. The method according to one of preceding claims 21 to 25, wherein the number of the precipitates is 1×10⁶/mm² or more.
 32. The method according to claim 21 or 25, wherein the P content is 0.015% or less.
 33. The method according to claim 21 or 25, wherein the P content is from 0.03 to 0.2%.
 34. The method according to claim 21 or 25, wherein the composition further comprises at least one kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.
 35. The method according to claim 21 or 25, wherein the composition further comprises 0.01 to 0.2% of Mo.
 36. The method according to claim 34, wherein the composition further comprises 0.01 to 0.2% of Mo.
 37. The method according to claim 22, 24 or 25; wherein the sum of Mn and Cu is from 0.08 to 0.4%.
 38. The method according to claim 22, 24 or 25, wherein the Mn content is from 0.01 to 0.12%.
 39. The method according to claim 22, 24 or 25, wherein the value of (Mn/55+Cu/63.5)/(S/32) is from 1 to
 9. 40. The method according to claim 23, 24 or 25, wherein the value of (Al/27)/(N/14) is from 1 to
 5. 